Shortly after your mentor's passing, you received an envelope along with the following note:

My Dear Disciple,

I am afraid my time to leave this world has come but don't let it bother you, for I have already lived a long and fulfilled life. As a final goodbye and an honour to all the precious moments we spent crafting and solving puzzles together, I have spent these last few months orchestrating this treasure-hunt just for you.

The key and the chest have been sent to my most faithful friend overseas and are now being kept/ buried in different locations. Every piece of paper in this envelope has all the clues you need to find the specific location of both the key and the chest.

This treasure is not only my final gift to you but also to your family. The fortune inside is more than enough to lay a foundation of a good life for your children. Let this treasure be the legacy that will pass on to their generation and the beginning of their endless opportunity that is to come.

Love,
Your Mentor

Inside the envelope are three zipper bags with a label attached; each bags contain pieces of paper with clues. One of the papers has been torn up to smaller pieces.

Here are all the pieces of paper found in each bags:

#

⌂⌂⌂, ⚑

⌂, ⌂⌂⌂, ⚑

Text:
Kb iochy uv bawmoj

Find the key and the chest.

Hint:

- The fish lost his toque blanche he usually wears.
- the fish = ⚑, cardinal directions = ⌂⌂⌂

Hint II (Hoping this will make the puzzle less of fumbling in the vast dark room.):

- ⚑ = country, ⌂⌂⌂ = city/ town/ village
- Mentor's favourite land is where the potatoes turn french. (This hint bypasses the fish by narrowing it down to a province)

Hint III:

Hint IV:

We’ve already established what country and province the key and chest would be in from @OmegaKrypton's answer. The next step should be finding out what ⌂⌂⌂s each cardinal direction represents.

Hint V:

- Save every image and inspect closely what distinguishes the white from brown paper.
- Google Maps is your friend.
- Although the last two papers possess the same quality as the second paper, they're not meant to be used with the same reference.
- Observe back and forth between all the pieces of paper and find a relation between them.

The rebus for N shows an incon for "undo" or "go back" and the back is the hind part; the letter e; a loop and the letter n. (And you can probably work out the second rebus in Hint III.) Hindeloopen is a small town on the IJsselmeer.

A few kilometres to the east is It Heidenskip and the fast-forward button in the E rebus can mean "skip". Workum (the map above shows the Frisian name Warkum) is to the north of both and the N rebus begins with W, but I'm not sure how the cog can mean something like "ark". The S rebus is completely covered, but I'm sure it is Koudum.

The paper with the diamond, circle and red X ...

... uses these four places as reference for the diamond, as Kami V found out. We can scale and turn that paper and overlay in on a map – thankfully, the image background is transparent. In the configuration below, the circle lines up with the ring road around Sneek:

... seems to require a key. One poular cipher with a key is the Vigenère cipher. There is a key symbol on the southern tip of the diamond at Koudom. Using that as key, the message decodes to:

An olivy uv bawmoj

(Only the bold parts have been decrypted, the rest is left as is.) That could be the start to a sentence, perhaps "an olive". To cut a long story short: I found the intended message more or less by accident, but the OP pointed me towards the intended solution, which I'll disclose now as if it were my own.

The piece of paper shows a diamond, which not only relates to the locations of the four towns, but also to the positions of the masking tape on the torn sheet. That sheet had the rebuses for the four towns:

W = Hindeloopen
S = KoudumN = Workum
E = It Heidenskip

The bold parts are covered by tape. (The cassette tape symbol in the clipping with the encrypted message is supposed to point towards the tapes. I didn't see that, because I failed to associate the cassette tape with the sticky tape.) The bold parts are also arranged in a diamond shape and the arrows point to the N, E and S parts, which yield the Vigenère key "koudum um it heiden" and the plain text:

An olive in itself

The key ...

... is hidden in Koudum. (Kami V saw that: The key symbol is in Koudum's place in the diamond.) The decoded message, "An olive in itself" refers to the exact location of the key in Koudum.

At the centre of Koudum is Martinikerk, the church of St Martin's. The church has an oval, grassy churchyard surrounding it, which looks like an olive on a map. And, of course, a Martini is usually served with an olive "in itself". The key is hidden in Martinikerk in Koudum.

The clipping with the volcano sketch ...

... has some sublte nuances of transparency / blackness: There are thin, dark lines and broader, slightly brigher lines. I've increased the brightness to make them stand out better:

This piece of paper can be overlaid over the torn paper:

The thick lines are all on the masking tape, perhaps indicating that it is difficult to write there with a felt tip. The folds in the tape line up nicely with the right flank of the volcano and the left bit of shore. Now look at the tears, which I have highlighted.

The treasure chest ...

... is hidden in Oudega. The tears in the torn papers are streets in Oudega. More precisely, the tear pattern match with the street map near Pastorijstrjitte:

The X in the overlain volcano map points to a sandpit in a playing ground. So the treasure chest is buried in the sandpit in a playing ground in Pastorijstrjitte in Oudega.

$\begingroup$The decrypted message is indeed correct. Great job on the work-around! But can you also solve it by the intended process? Because, it might give you a clue on what to do with the volcano paper (slightly similar method). HINT: Did you notice that the arrangement of the arrows in the last paper are suspiciously similar to another detail in the puzzle? Maybe that's the reference you're supposed to overlay it with. And for the cassette symbol, what does it alternatively called—like a very short word of it?$\endgroup$
– ChroneFeb 7 at 14:00

$\begingroup$Really? I didn't expect that. I know that I can overlay the tape paper on the diamond sketch without rotating. The actual key I used is "Koudum um It Heide(nskip)", so I'm confused about where the "um" comes from. The phrase might mean "to K via IH", but "um" doesn't seem to ba a Dutch word. And a music cassette is a tape ot an MC. Hmmm.$\endgroup$
– M OehmFeb 7 at 14:41

$\begingroup$The arrows are not pointing at the towns, they refer to a position of something else that’s arranged like a shape of diamond indicated by the symbol at the starting point. Can you see it :)$\endgroup$
– ChroneFeb 7 at 14:57

$\begingroup$The puzzle has some very nice elements. It's a pity that it didn't get much attention. This seems like a good puzzle to solve in a team -- someone in the team would probably have seen the things I've missed or got wrong.$\endgroup$
– M OehmFeb 8 at 7:40

is a red herring with a knife with the words "stainless steel" as suggested by @MOehm in the comments. From the hint perhaps the fish is a cook, due to the toque blanche, which is the white hat chefs wear

:: Lower image

Area covered by tape ii is

clearly the remaining part of the letter N, therefor the letters on the LHS are W, S, N, and E, the four directions

$\begingroup$The cipher is incorrect. You’re right to be skeptical about the rot13(erq ureevat) but your interpretation is off the mark. The letters on the left of :: lower image are indeed four directions.$\endgroup$
– ChroneJan 15 at 11:02

$\begingroup$“Stainless steel” is unimportant otherwise it would be a very mean thing to do—making a clue so small to the point of being unreadable. The fish is not necessary needed to solve the puzzle but it’s also a valid clue that if solved, make the puzzle a little more easier. Because, If you know the ⌂⌂⌂ then the ⚑ is not necessary needed anymore but it’s much more easier to know the ⚑ first and then find the ⌂⌂⌂ in it.$\endgroup$
– ChroneJan 17 at 9:25

$\begingroup$Your take on "mentor's favourite land" is very very close but, unlike the fish, It doesn’t refer to a country. You got all the words in there, the tail and the head. Just have to connect it.$\endgroup$
– ChroneJan 21 at 6:42

$\begingroup$Ha! That's a good observation, but I think the scale is a bit off. Playing with your idea a bit, I get this, which would place the X at Oudega. I've no idea what I should make of the other piece of paper with an X, though. I've also thoght about Koudum as key, but in another way, but it only gets me halfway there. Maybe there's a connection with note with the encrypted text?$\endgroup$
– M OehmFeb 3 at 19:32

1

$\begingroup$@MOehm That’s spot on! Regarding the note with the encrypted text, maybe the new hint I’ve added can help you.$\endgroup$
– ChroneFeb 5 at 10:02